1. Field of the Invention
The present invention relates to a thin-film EL (electroluminescent) display panel used in a display unit of various types of information terminals and for an indicator mounted in cars, and more particularly to a thin-film EL display panel structured by laminating two thin-film EL elements and to a method of fabricating the same.
2. Description of the Related Art
A thin-film EL display panel utilizes a phenomenon whereby light is emitted when an electric field is applied to a phosphor having zinc sulfide (ZnS) or the like as its base material.
Luminescent colors of this type of thin-film EL display panel may be changed in various ways by changing the type of luminescent central elements doped within the luminescent layer. For example, when manganese (Mn) is doped into zinc sulfide (ZnS) as the luminescent base material, the luminescent layer emits orange-colored light. It also emits green, red, blue and white light, respectively, when terbium fluoride (TbF.sub.3), samarium chloride (SmCl.sub.3), thulium chloride (TmCl.sub.3) and praseodymium fluoride (PrF.sub.3) are doped into ZnS.
Then, a thin-film EL display panel in which thin-film EL elements, each of which emits a different color, are formed on two substrates, wherein at least one substrate is transparent, and the EL elements are laminated and bonded to allow the device to display varying colors has been proposed (see, e.g., Japanese Patent Publication Laid-Open No. Sho. 59-133584).
Because this variable color thin-film EL display panel may be constructed simply by laminating monochromatic double insulating type thin-film EL elements, its structure is relatively simple. Furthermore, because the EL elements, each having a different luminescent color, may be selected and checked before final assembly, the yield of the product is good and its reliability is high.
While the thin-film EL display panel is generally apt to deteriorate due to airborne moisture and the like, in order to protect it, the whole EL element is sealed by silicon oil or the like. The variable color thin-film EL display panel described above has also another advantage in that it requires no dummy substrate for sealing because the EL elements are laminated while facing each other and silicon oil or the like may be sealed in the space formed therebetween.
However, the thin-film EL display panel constructed by laminating two thin-film EL elements has a problem as described below. Because lead wires have to be connected to connecting terminal portions of electrodes on each separated substrate, the packaging and assembling process including the lead connection becomes very complicated and cumbersome when a large number of connecting terminal portions are provided at the periphery of the substrate. Due to that, a lead connection structure for a thin-film EL display panel structured by laminating two thin-film EL elements together was proposed in Japanese Patent Publication Laid-Open No. Sho. 64-60993.
As shown in FIG. 25, in the above-described prior art thin-film EL display panel, each of connecting terminals 92 and 93 is connected to a lead wire member 94 by providing the terminal portions 92 and 93 of each electrode of two thin-film EL elements 90 and 91 at the periphery of the elements, laminating both thin-film EL elements 90 and 91 to form a very small gap therebetween and inserting each lead wire member 94, which may be, for example, a flexible printed circuit board (FPC), in the small gap with layers of electrical insulation disposed between opposed lead wire members 94 as shown in FIG. 25. However, the width of the gap between the two thin-film EL elements 90 and 91 can be as small as about 50 .mu.m, and it is actually impossible to connect the lead wire member 94 after laminating the two thin-film EL elements 90 and 91 together.
Due to that, although it is necessary to connect the lead wire members 94 to each connecting terminal portion 92 and 93 before laminating the EL elements 90 and 91 together, it is very difficult to accurately position and bond the two substrates 90 and 91 together after attaching the lead wire members 94. Furthermore, when silicon oil fills the gap between the EL elements 90 and 91 to prevent moisture after that, the oil adheres to the lead wire member 94 and it is difficult to clean it.
Furthermore, when an FPC is used as the lead wire member 94, although it is necessary to widen the gap between the two thin-film EL elements 90 and 91 from 200 .mu.m to 400 .mu.m in order to dispose two FPCs in the gap since the thickness of the board is normally 100 .mu.m to 200 .mu.m, there has been a problem in that when both thin-film EL elements 90 and 91 are bonded together while widening the gap therebetween, the displayed color of the variable color display is likely to blot or blur, thereby degrading the display quality.
Meanwhile (although this technique is not prior art to the present invention), in the case of a dot-matrix type thin-film EL display panel, it is possible to laminate and bond two EL elements 95 and 96 together while shifting them and to connect the lead wire member after sealing with silicon oil as shown in FIG. 26 in order to avoid the problem of the connection of the lead wire member described above.
In the case of such a thin-film EL display panel, however, because connecting terminal portions 97 and 98 of each strip electrode of each of the thin-film EL elements 95 and 96 are located on one side of the electrode, the distance from each light emitting display dot (the intersection of the strip electrodes) 99 to each of the connecting terminal portions 97 and 98 largely differs depending on the position of each display dot.
Due to that, when using a transparent electrode material such as indium-tin oxide (ITO) having a relatively large resistance, a voltage and current between the connecting portions 97 and 98 and the display dot are large near the connecting terminal portions 97 and 98 and the farther the distance therefrom, the lower the current and voltage between the electrodes becomes, causing nonuniformity of brightness on the display screen of such a thin-film EL display panel.
Furthermore, in the case of the variable color thin-film EL display panel in which two thin-film EL elements each having a different luminescent color are laminated together, because the luminescent color of each element is controlled by changing a voltage signal or the like applied to each thin-film EL element and an attempt is made to display a predetermined color, the composite display color varies depending on the position of a particular pixel on the display screen, thus causing nonuniformity of the overall display color.